Permeability separatory devices in which the membrane takes the form of permeable hollow fibers are now well known. Such devices offer a high membrane area per unit volume of module and do not require membrane supporting means, even when trans-membrane pressure differentials of up to 1500 rounds per square inch are employed. However, such units have not been free of support-connected problems.
It is conventional practice to pot the ends of the hollow fibers in a solid body of resin which is conveniently referred to as a tubesheet. The face of the tubesheet is sliced off (or protruding fiber loops are cut off) to permit egress of permeate (water, for example) from the fiber lumens when the fiber/tubesheet assembly is placed, together with suitable sealing means, in a pressurizeable casing and a fluid (an aqueous brine, for example) is supplied to the exterior of the unpotted fiber portions under sufficient pressure to cause permeation through the fiber walls. The pressure differential, between the back of the tubesheet--from which the unpotted fiber portions extend--and the tubesheet face, exerts a force which can be very substantial (as in reverse osmosis processes, for example). This force tends to deform the tubesheet and results in shear stresses which can lead to failure. Thus, some means of supporting the tubesheet is generally required, even at the expense of increased resistance to permeate egress from the fiber lumens.
Perhaps the simplest prior art tubesheet support means is a perforated metal plate as disclosed in U.S. Pat. No. 3,422,008.
An alternative support means has been disclosed (for tubesheets not located at the ends of fiber bundles) in U.S. Pat. Nos. 3,455,460 and 3,475,331. The '460 patent is directed to a type of permeator in which the hollow fibers are spirally wound, in layers, around an inner, elongated core and the tubesheet is disposed longitudinally, like a dike, rising from the core to the bundle periphery and extending from one end to the other of the bundle. The fibers passing through the tubesheet are opened by routing out a trench (into which a porous support member is inserted) or by drilling holes (which may or may not penetrate the core wall). The '331 patent discloses a spherically wound hollow fiber bundle with an equatorial tubesheet (dike) which is drilled or routed to open the fiber lumens.
Another solution to the support problem is disclosed in U.S. Pat. No. 4,061,574. The tubesheet is opened for permeate egress by a plurality of bores which are generally parallel to the tubesheet face and connect with an axial conduit or a peripheral, annular conduit. The end of the tubesheet is positioned directly against (and supported by) the casing end.
By using a central, radially bored tubesheet, as disclosed in U.S. Pat. No. 4,080,296, the permeator may be otherwise designed and so operated that a "pressure balanced" condition results. That is, the pressures on the opposite faces of the tubesheet are essentially equal and "support" resides in the compressive strength of the bored tubesheet structure itself.
The nearest art to the present invention known of is the support method disclosed in U.S. Pat. No. 3,702,658. A porous body of a non-compressible material, such as sand beds or ceramic or metal frits, is disposed between the casing end and the tubesheet. Reduced resistance to permeate flow is said to result from using materials such that the solid segments of the support surface of the body are from 10 to 30 times as wide as the fiber lumen diameter, the total area of the segments is from about 30 to 70% of the total surface area and the pores making up the rest of that area have average diameters of from 1 to 3 times the fiber lumen diameter.
Of those of the foregoing methods which utilize foraminous supporting members, at least some are currently in use. Even the latter, however, need improvement as to cost effectiveness, ease of scale-up, corrosion resistance, and ease of cleaning for re-use. Also, the contact area between the tubesheet face and the solid portions of the support surface generally cannot be estimated with the accuracy desirable for design purposes.